Embodiments of the invention relate generally to heated tip fuel injectors, and more particularly, to controlling and driving an induction-heated fuel injector.
There is a continued need for improving the emissions quality of internal combustion engines. At the same time, there is pressure to minimize engine crank times and time from key-on to drive-away, while maintaining maximum fuel economy. Those pressures apply to engines fueled with alternative fuels such as ethanol as well as to those fueled with gasoline.
During cold temperature engine start, the conventional spark ignition internal combustion engine is characterized by high hydrocarbon emissions and poor fuel ignition and combustibility. Unless the engine is already at a high temperature after stop and hot-soak, the crank time may be excessive, or the engine may not start at all. At higher speeds and loads, the operating temperature increases and fuel atomization and mixing improve.
During an actual engine cold start, the enrichment necessary to accomplish the start leaves an off-stoichiometric fueling that materializes as high tail-pipe hydrocarbon emissions. The worst emissions are during the first few minutes of engine operation, after which the catalyst and engine approach operating temperature. Regarding ethanol fueled vehicles, as the ethanol percentage fraction of the fuel increases to 100%, the ability to cold start becomes increasingly diminished, leading some manufacturers to include a dual fuel system in which engine start is fueled with conventional gasoline and engine running is fueled with the ethanol grade. Such systems are expensive and redundant.
Another solution to cold start emissions and starting difficulty at low temperature is to pre-heat the fuel to a temperature where the fuel vaporizes quickly, or vaporizes immediately (“flash boils”), when released to manifold or atmospheric pressure. Pre-heating the fuel replicates a hot engine as far as fuel state is considered.
A number of pre-heating methods have been proposed, most of which involve preheating in a fuel injector. Fuel injectors are widely used for metering fuel into the intake manifold or cylinders of automotive engines. Fuel injectors typically comprise a housing containing a volume of pressurized fuel, a fuel inlet portion, a nozzle portion containing a needle valve, and an electromechanical actuator such as an electromagnetic solenoid, a piezoelectric actuator or another mechanism for actuating the needle valve. When the needle valve is actuated, the pressurized fuel sprays out through an orifice in the valve seat and into the engine.
One technique that has been used in preheating fuel is to inductively heat metallic elements comprising the fuel injector with a time-varying magnetic field. Exemplary fuel injectors having induction heating are disclosed in U.S. Pat. No. 7,677,468, U.S. Patent Application No's: 20070235569, 20070235086, 20070221874, 20070221761 and 20070221747, the contents of which are hereby incorporated by reference herein in their entirety. The energy is converted to heat inside a component suitable in geometry and material to be heated by the hysteretic and eddy-current losses that are induced by the time-varying magnetic field.
The inductive fuel heater is useful not only in solving the above-described problems associated with gasoline systems, but is also useful in pre-heating ethanol grade fuels to accomplish successful starting without a redundant gasoline fuel system.
Because the induction heating technique uses a time-varying magnetic field, the system includes electronics for providing an appropriate high frequency alternating current to an induction coil in the fuel injector.
Conventional induction heating is accomplished with hard-switching of power, or switching when both voltage and current are non-zero in the switching device. Typically, switching is done at a frequency near the natural resonant frequency of a resonator, or tank circuit. The resonator includes an inductor and capacitor that are selected and optimized to resonate at a frequency suitable to maximize energy coupling into the heated component.
The natural resonant frequency of a tank circuit is fr=1/(2π√{square root over (LC)}), where L is the circuit inductance and Cis the circuit capacitance. The peak voltage at resonance is limited by the energy losses of the inductor and capacitor, or decreased quality factor, Q, of the circuit. Hard-switching can be accomplished with what are called half-bridge or full-bridge circuits, comprising a pair or two pairs of semiconductor switches, respectively. Hard-switching of power results in the negative consequences of switching noise, and high amplitude current pulses at resonant frequency from the voltage supply, or harmonics thereof. Also, hard switching dissipates power during the linear turn-on and turn-off period when the switching device is neither fully conducting nor fully insulating. The higher the frequency of a hard-switched circuit, the greater the switching losses.
The preferred heater circuit therefore provides a method of driving a heated fuel injector wherein switching is done at the lowest possible interrupted power. This heater circuit was disclosed in U.S. Pat. No. 7,628,340, Title: Constant Current Zero-Voltage Switching Induction Heater Driver for Variable Spray Injection. Ideally, energy should be replenished to the tank circuit when either the voltage or the current in the switching device is zero. It is known that the electromagnetic noise is lower during zero-voltage or zero-current switching, and is lowest during zero-voltage switching, this is the method of U.S. Pat. No. 7,628,340. It is also known that the switching device dissipates the least power under zero switching. That ideal switching point occurs twice per cycle when the sine wave crosses zero and reverses polarity; i.e., when the sine wave crosses zero in a first direction from positive to negative, and when the sine wave crosses zero in a second direction from negative to positive.
It is preferable to reduce the size of inductive components and in some cases, eliminate the impedance-matching transformer, while maintaining the minimum necessary connections to the inductive heater coil on the injector. The difficulties of this will be detailed in this specification in subsequent text. Embodiments of the invention continue to provide for the elimination of the hard-switching and its negative consequences, replace it with zero-voltage switching, and further apply this method in a full-bridge topology while advantageously eliminating the impedance matching transformer and overcoming the difficulties of alternative solutions.
Referring to FIG. 3, the description disclosed in U.S. Pat. No. 7,628,340 includes a center-tapped impedance matching transformer that those skilled in the art will be familiar with as to the additional cost this topology might present.
Referring to FIG. 4, elimination of the impedance matching transformer is possible and practical, but with the result that the induction heater coil uses a center-tap to satisfy the push-pull oscillator topology. This center-tap adds an additional conductor and pin to the inductively heated fuel injector.
Referring to FIG. 5, the further elimination and change of the topology of the push-pull oscillator follows from the FIG. 4 topology, however, it uses two constant-current inductors of a relatively much higher inductance compared to the induction heater coil to allow sufficient current to flow through the impedance of the heater coil, otherwise the majority would flow only through the power switches. In order to maintain sufficient heater current, this tends to result in a small inductance of the heater coil rather than a reduction of constant-current inductance. This reduction of heater coil inductance tends to the point that parasitic inductance of the wire harness and connections may overwhelm that of the heater coil. Additionally, in order to obtain a small heater coil inductance, the number of coil turns is reduced for a given magnetic mass available, possibly sacrificing Ampere-Turns per Hertz and thereby detrimentally affecting the ability to perform induction heating.